1. Field of the invention
The present invention relates to a method for converting radiographic images, and more particularly to a method for the converting radiographic images which utilizes a stimulable phosphor.
2. Description of the state of the art
In order to obtain radiographic images, there has until now been used silver halide-utilizing photographic system, the so-called radiographic photography, but in recent years there has arised a demand for the development of method for the formation of radiographic images without use of a silver halide because of the global shortage of silver resources.
As a method substitutable for the above-mentioned radiographic photography, there has now been contemplated such a method that the radiation that has passed through an object is absorbed into a phosphor, and after that, the phosphor is excited by a certain energy to thereby radiate the radiation energy stored in the phosphor as fluorescence, and the fluorescence is then detected to thereby form an image. To be concrete, there is proposed a method for the converting radiographic images wherein as the phosphor, a thermoluminescent phosphor is used, and as the excitation energy, thermal energy is used (British Pat. No. 1,462,769 and Japanese Pat. Publication Open to Public Inspection (hereinafter referred to as Japanese Pat. O.P.I. Publication) No. 29889/1976). This conversion method is such that a panel comprising a support formed thereon with a thermoluminescent phosphor layer is used, and into the thermoluminescent phosphor layer of the panel is absorbed a radiation that has passed through an object to thereby store therein a radiation energy pattern corresponding to the pattern of the radiation, and after that the thermoluminescent phosphor layer is heated to take out the stored radiation energy in the form of a light signal pattern, the light pattern forming an image. However, it is indispensable for this method that the panel is so heat-resistant as not to be deformed nor to be changed in quality by heat because it must be heated at the time of changing the stored radiation energy into a light signal, and therefore large restrictions are put on the materials used for the thermoluminescent phosphor layer and the support which are the components of the panel. Thus, there exists a large drawback in the practical use of the method which uses a thermoluminescent phosphor as the phosphor and thermal energy as the excitation energy. On the other hand, there are also known other methods which use at least one of visible rays and infrared rays as the excitation energy as described in U.S. Pat. No. 3,859,527, and Japanese Pat. O.P.I. Publication Nos. 12142/1980, 12143/1980, 12144/1980, 12145/1980, 84389/1980, 160078/1980, and the like.
These methods need not be heated at the time of changing the stored radiation energy into a light signal unlike the previously mentioned method, and therefore the panel need not be heat-resistant, so that the methods may be deemed favorable for the conversion of radiographic images.
However, among those phosphors used in the methods disclosed in the above-mentioned publications, for example, cerium- and samarium-activated strontium sulfide phosphor (SrS:Ce,Sm) europium- and samarium-activated strontium sulfide phosphor (SrS:Eu,Sm), europium- and samarium-activated lanthanum oxysulfide phosphor (La.sub.2 O.sub.2 S:Eu,Sm), manganese- and halogen-activated zinccadmium sulfide phosphor [(Zn,Cd)S:Mn,X wherein X is a halogen], europium-activated barium aluminate phosphor, alkaline earth metal silicate phosphor, rare earth element-activated lanthanum oxyhalide phosphor, and the like, are extremely low sensitive, so that from the practical application point of view, the improvement on the sensitivity in the formation of radiographic images had been sought. Further, among those phosphors used in the above methods, copper- and lead-activated zinc sulfide phosphor (ZnS:Cu,Pb), europium-activated alkaline earth fluorohalide phosphor, rare earth element-activated alkaline earth metal fluorohalide-type phosphor, rare earth element-activated divalent metal fluorohalide phosphor, and the like, have extremely low response speed to the excitation light that is used for releasing the stored radiation energy as fluorescence (hereinafter referred to as "stimulating ray or light", and therefore they have the disadvantage that the speed thereof to read out the image recorded in the phosphor becomes extremely reduced.